The aim of the propsed work is to measure segmental motions of the membrane Ca2 ion/Mg2 ion-ATPase of skeletal muscle sarcoplasmic reticulum. This will be done by measuring the distance between two fluorescent probes, attached at unique sites on the enzyme, using the technique of fluorescence energy transfer. Differences in the distance between the probes will be determined as the ATPase is trapped intermediate conformations along the kinetic pathway of ATP hydrolysis and concurrent Ca2 ion translocation.